i now leave a little free room on my iPod (about 1GB)
a while ago if my iPod was full, the next sync would always throw up an error (unknown error number -422 or something) and i would have to do a complete restore, and a complete re sync, never found out what the problem was or if its now fixed, its just better for me to leave a little spare room on the iPod than to have to sync 30GB of music and films just to put on a podcast or a new album.
granted this was about 6months ago, just got into the habit of leaving some space.